As the density of semiconductor devices increases and the size of circuit elements becomes smaller, the resistance capacitance (RC) delay time increasingly dominates the circuit performance. To reduce the RC delay, there is a desire to switch from conventional dielectrics to low-k dielectrics. These materials are particularly useful as intermetal dielectrics, IMDs, and as interlayer dielectrics, ILDs. However, low-k materials present problems during processing, especially during the processing of the conductive material used to make interconnects.
The conductive material is typically patterned and etched using high-energy plasma etch processes. The low-k materials are susceptible to damage from a plasma etch because they are softer, less chemically stable or more porous, or any combination of these factors. The plasma damage can manifest itself in higher leakage currents, lower breakdown voltages, and changes in the dielectric constant associated with the low-k dielectric material.
One example of a low-k material is a carbon-doped oxide or organosilicate glass (OSG). OSG films typically comprise SiwCxOyHz, wherein the tetravalent silicon may have a variety of organic group substitutions. A commonly used substitution creates methyl silsesquioxane (MSQ), wherein a methyl group creates a SiCH3 bond in place of a SiO bond. Upon exposure to a processing plasma, as in photoresist removal, plasma damage may cause the methyl group to be replaced with a OH group, thereby forming a silanol.
Silanol bonds at the surface of the OSG material have been observed to degrade the integrity of a low-k dielectric film. One form of degradation is the increase in the dielectric constant of the low-k dielectric material due to the presence of the silanol. In addition, the damaged OSG material has been observed to adsorb moisture. It has also been observed that this degraded low-k dielectric material is vulnerable to chemical attack during exposure to wet chemical cleanups, which results in significant critical dimension (CD) loss of low-k dielectric film insulating structures.
Another example of low-k dielectric materials are the porous dielectrics such as the commercially available Dow Chemical's porous SILK product and JSR Corporation's JSR 5109. The dielectric constant of the porous material is a combination of the dielectric constant of air and the dielectric constant of the dense material. Silica based xerogels and aerogels, for example, incorporate a large amount of air in pores or voids, thereby achieving dielectric constants less than 1.95 with pores as small as 5–10 nm.
Just as with carbon containing dielectrics, porous dielectrics are also susceptible to damage from plasma etching and ashing processes used in device fabrication. When there is an open pore in the dielectric, processing fluids in lap and polish and in thin film metallization can enter surface pores, thereby causing corrosion, mechanical damage, or an increase in the dielectric constant. Pore damage may also cause a surface that is preferably hydrophobic to become hydrophilic.
FIG. 1 shows a schematic representation of a cross-section of a damascene structure. Dielectric 12 has been laid down over wiring level 11. Extending downwards from trench 15 is a via 14. When the structure has been filled with copper, via 14 provides a connection between the two wiring levels. Because of its high diffusivity and its tendency to act as a recombination center in silicon, steps must be taken to ensure that all the copper is confined to the damascene structure. This is conventionally accomplished with a barrier layer 18 in that lines the walls of the trench 15 and via 14, as illustrated in FIG. 2.
As described above, chemicals may penetrate into the low-k dielectric pores and raise its dielectric constant. Low-k dielectric damage also causes surface roughness of the trench floor 21 and trench wall 23, as shown in FIGS. 1 and 2. The rougher surface means that a much thicker barrier layer 18, FIG. 2, than normal is needed to ensure that there are no thin patches through which copper could move. The thicker barrier layer 18, in turn, partially offsets the advantage of the low-k dielectric by increasing the resistance capacitance (RC) delay time.
As noted above, dielectric damage causes higher leakage currents, lower breakdown voltages, and changes in the dielectric constant associated with the low-k dielectric material. In view of these and other problems, there is a need for improved low-k dielectric manufacturing methods.